Oxide garnet single crystal

ABSTRACT

A novel oxide garnet single crystal, which can be epitaxially grown on a rare earth-gallium garnet wafer and exhibiting excellent performance as a material of magneto-optical devices, is disclosed. The oxide garnet single crystal, grown on the substrate surface by the liquid-phase epitaxial method, has a chemical composition expressed by the formula 
     
         (Bi.sub.a Eu.sub.b Ln.sub.1-a-b).sub.3 (Fe.sub.1-c M.sub.c).sub.5 O.sub.12, 
    
     in which Ln is a rare earth element other europium, e.g. terbium, M is an element selected from the group consisting of aluminum, gallium, indium, and scandium, the subscript a is a positive number defined by 0.15≦a≦0.6, the subscript b is a positive number defined by 0.01≦b≦0.2 and the subscript c is a positive number defined by 0.01≦c.≦0.1.

BACKGROUND OF THE INVENTION

The present invention relates to a single crystal of an oxide garnet or,more particularly, to a novel single crystal of an oxide garnet usefulas a material of magneto-optical devices such as optical isolators,optical switches and the like exhibiting improved behavior of lightabsorption at the working wavelengths.

It is an established prior art that magneto-optical devices such asoptical isolators and the like are prepared from a single crystal layerof a bismuth-substituted rare earth-iron garnet grown on a substratesingle crystal by the liquid-phase epitaxial method. There is anunavoidable problem in this liquid-phase epitaxial method that thegarnet single crystal is heavily contaminated with lead or platinumoriginating in the lead oxide used as a flux of the oxide melt and theplatinum-made crucible, respectively. When such a contaminated epitaxialgarnet single crystal is used as the magneto-optical element, absorptionof light is greatly increased at the working wavelengths of 0.8 μm, 1.3μm and 1.55 μm to cause an increase in the insertion loss.

A remedial measure for the above mentioned problem is proposed inPreprint for the 11th Conference of Japan Association of AppliedMagnetics, Nov. 1987, 2C-10, page 137, according to which trace amountsof divalent or tetravalent metallic ions, e.g., Ca²⁺, Mg²⁺ and Ti⁴⁺, areadded to the oxide garnet single crystal of this type. This method,however, does not provide a complete solution of the problem because theaddition of the above mentioned metallic ions to the melt for theepitaxy necessarily causes changes in the composition of the epitaxiallygrown layer in the course of the growth so that the uniformity in theperformance of the epitaxial garnet layer throughout the thickness canhardly be ensured since the epitaxial layer is usually required to havea thickness as large as 50 μm or larger.

SUMMARY OF THE INVENTION

The present invention accordingly has an object to provide an oxidegarnet single crystal free from the above described problems anddisadvantages in the prior art.

Thus, the present invention provides an oxide garnet single crystalhaving a chemical composition expressed by the formula

    (Bi.sub.a Eu.sub.b Ln.sub.1-a-b).sub.3 (Fe.sub.1-c M.sub.c).sub.5 O.sub.12,(I)

in which Ln is a rare earth element other than europium or, preferably,terbium, M is an element selected from the group consisting of aluminum,gallium, indium and scandium or, preferably, gallium, the subscript a isa positive number defined by 0.15≦a≦0.6, the subscript b is a po-sitivenumber defined by 0.01≦b≦0.2 and the subscript c is a positive numberdefined by 0.01≦c≦0.1.

The above defined oxide garnet single crystal is preferably formed as anepitaxial layer on a substrate wafer of neodymium gallium garnet orgadolinium gallium garnet partially substituted with calcium, magnesiumand/or zirconium having a specified lattice constant by the liquid-phaseepitaxial method.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a photograph showing the maze-like magnetic domain structurein the oxide garnet single crystal prepared in Example 1 of theinvention taken with a polarizing microscope.

FIG. 2 is a schematic diagram illustrating the optical system for themeasurement of the magneto-optical characteristics of the inventiveoxide garnet single crystal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As is described above, the oxide garnet single crystal of the inventionis characterized by the unique chemical composition expressed by theabove given formula (I). The oxide garnet is a kind of bismuthrare-earth iron garnet of which europium is an essential element as therare earths. This unique chemical composition has been discovered as aresult of the extensive investigations undertaken by the inventors for arare earth element which can be in the divalent or tetravalent state inthe bismuth rare-earth iron garnet. According to the discovery, europiumhas an effect to greatly improve the optical properties of the garnetcrystal relative to the absorption of light. Being a rare earth element,furthermore, europium of course has no problem in the compatibility withthe other rare earth elements so that the problem of non-uniformdistribution of the element is rarely caused throughout the epitaxiallayer however thick it may be as produced by the liquid-phase epitaxialmethod.

The oxide garnet single crystal of the invention is characterized by thechemical composition expressed by the formula

    (Bi.sub.a Eu.sub.b Ln.sub.1-a-b).sub.3 (Fe.sub.1-c M.sub.c).sub.5 O.sub.12.(I)

In the formula, Ln is a rare earth element other than europium or,namely, an element selected from the elements having atomic number of39, 57 through 62 and 64 through 71. The element of the atomic number61, i.e. prometheum, may be excluded in view of non-occurrence thereofin nature. It is optional that two kinds or more of these rare earthelements are used as the Ln in combination according to need. M is anelement selected from the group consisting of aluminum, gallium, indiumand scandium. The addition of the element M to the composition ispreferably in respect of the decrease in the saturation magnetization sothat the magneto-optical device can be more compact. The subscripts aand b are each a positive number in the ranges of 0.15 to 0.6 and 0.01to 0.2, respectively, and the subscript c is a positive number in therange from 0.01 to 0.1 or, preferably, in the range from 0.05 to 0.1.

The oxide garnet single crystal of the invention can be obtained usuallyin the form of an epitaxial layer grown on the surface of a substratewhich is preferably a single crystal of a rare earth gallium garnet. Theepitaxial layer is formed by the well-known liquid-phase epitaxialmethod from a melt of a mixture of the oxides of the elements formingthe resulting oxide garnet and a flux ingredients kept at a temperaturesomewhat lower than the melting point of the mixture.

The rare earth-gallium garnet used in the form of a single crystal waferas the substrate on which the specified oxide garnet single crystal isgrown by the liquid-phase epitaxial method is preferably samariumgallium garnet, referred to as SGG hereinbelow, neodymium galliumgarnet, referred to as NGG hereinbelow, or gadolinium gallium garnet,referred to as GGG hereinbelow, partially substituted with one or moreof the elements of calcium, magnesium and/or zirconium. Such asubstituted GGG is available as a commercial product of the tradename ofSOG or NOG (products by Shin-Etsu Chemical Co.). The rare earth-galliumgarnet single crystals can be prepared by the Czochralski method from amelt of a mixture of the respective oxides. The epitaxial growth of theoxide garnet single crystal by the liquid-phase epitaxial method isperformed by heating and melting a mixture of bismuth oxide Bi₂ O₃,europium oxide Eu₂ O₃, oxide of the rare earth element Ln, iron oxideFe₂ O₃ and oxide of the element M with admixture of the flux ingredientssuch as lead oxide PbO and boron oxide B₂ O₃ at a temperature of 1100 °to 1200° C. and keeping the melt in a supercooled condition at atemperature of 750° to 950° C. so that the epitaxial layer of the oxidegarnet single crystal is grown on the surface of the substrate waferkept in the melt.

It is important that the inventive oxide garnet single crystal grown inthe form of an epitaxial layer having a substantial thickness on thesurface of a substrate wafer is free from cracks or unduly high stress.In this regard, it is important that a good matching within ±0.0005 nmor, preferably, ±0.0003 nm is obtained between the lattice constants ofthe rare earth-gallium garnet as the substrate and the oxide garnetsingle crystal grown thereon in the form of an epitaxial layer. Thismatching in the lattice constants can be obtained by adequatelyselecting the values of the subscripts a, b and c in the formula (I)depending on the kinds of the elements. For example, the latticeconstant of the epitaxially grown layer of the inventive oxide garnetsingle crystal can be 1.2508±0.0005 nm and 1.2496±0.0005 nm when thesubstrate is NGG or NOG, respectively. When the chemical composition isadequately selected, the discrepancy between the lattice constants ofthe substrate and the epitaxial layer can be minimized so that theepitaxial layer of the inventive oxide garnet single crystal can be freefrom the defects of cracks, pits and the like regardless of thethickness thereof.

Furthermore, the europium oxide as an essential ingredient has goodcompatibility with the other ingredients so that the epitaxial layer ishighly uniform in the composition throughout its thickness however thickthe epitaxial layer may be. The europium ingredient serves to reduce theabsorption of light in the working wavelengths so that the epitaxialwafer according to the invention can be an excellent material of variouskinds of magneto-optical devices such as optical isolators.

In the following, the oxide garnet single crystal of the invention andthe epitaxial wafer are described in more detail by way of examples.

EXAMPLE 1

The substrate for the epitaxial growth was a wafer of NOG having adiameter of 3 inches and a thickness of 500 μm, of which the latticeconstant was 1.2496 nm. A melt of an oxide mixture was prepared from977.38 g of bismuth oxide Bi₂ O₃, 1.489 g of europium oxide Eu₂ O₃,13.929 g of terbium oxide Tb₄ O₇, 123.86 g of iron oxide Fe₂ O₃, 5.273 gof gallium oxide Ga₂ O₃, 936.36 g of lead oxide PbO and 41.77 g of boronoxide B₂ O₃ by heating at 1100° C. in a platinum crucible containing thesubstrate wafer. The temperature of the melt was gradually decreased andthe melt was kept at a temperature of 749.5° to 752.5° C. for 36 hours.The substrate wafer taken out of the melt after solidification was foundto be covered with an epitaxially grown layer of an oxide garnet singlecrystal having a thickness of 0.492 mm.

The thus formed epitaxial layer was analyzed by the ICP (inductivelycoupled plasma) emission spectrophotometric analysis to give a resultthat the composition thereof could be expressed by the formula (Bi₀.26Eu₀.07 Tb₀.67)₃ (Fe₀.94 Ga₀.06)₅ O₁₂. This oxide garnet single crystalafter removal of the substrate was polished on the surfaces and then cutinto a piece of 2.9 mm by 2.9 mm square having a thickness of 0.381 mm,which was examined on a polarizing microscope to find a pattern ofmagnetic domains as illustrated in FIG. 1. This photograph clearlyindicates that the oxide garnet single crystal has a very uniformstructure.

The test piece was then provided with an AR(anti-reflection) coating andfurther examined for the magneto-optical characteristics at a wavelengthof 1.3 μm using a measurement system schematically illustrated in FIG. 2having an LD (laser diode) light source 1, optical fibers 2, collimatorlens 3, polarizer 4, a pair of electromagnets 5, between which the testpiece 9 was inserted, analyzer 6 and photodetector 7, in which thediameter of the light beam was 1.0 to 1.5 mm and the intensity of theincident light from the LD light source 1 was 0.598 mW at thewavelengths of 1317 nm. The results of the magneto-optical measurementwere: angle of Faraday rotation 44.7°; loss by light absorption 0.03 dB;and temperature dependency of the angle of Faraday rotation 0.04°/°C.

For comparison, the same magneto-optical measurement as above wasconducted with a single crystal of another bismuth-rare earth-iron oxidegarnet having a chemical composition expressed by the formula (Bi₁.15Gd₀.55 Lu₁.30)Fe₅ O₁₂. The result was that the loss by light absorptionwas as large as 0.8 dB.

EXAMPLE 2

The substrate for the epitaxial growth was a wafer of NOG having adiameter of 3 inches and a thickness of 500 μm, of which the latticeconstant was 1.2496 nm. A melt of an oxide mixture was prepared from232.12 g of bismuth oxide Bi₂ O₃, 0.328 g of europium oxide Eu₂ O₃,2.051 g of yttrium oxide Y₂ O₃, 29.49 g of iron oxide Fe₂ O₃, 1.255 g ofgallium oxide Ga₂ O₃, 223.33 g of lead oxide PbO and 9.952 g of boronoxide B₂ O₃ by heating at 1100° C. in a platinum crucible containing thesubstrate wafer. The temperature of the melt was gradually decreased andthe melt was kept at a temperature of 745.0° to 748° C. for 17 hours.The substrate wafer taken out of the melt after solidification was foundto be covered with an epitaxially grown layer of an oxide garnet singlecrystal having a thickness of 0.350 mm.

The thus formed epitaxial layer was analyzed by the ICP (inductivelycoupled plasma) emission spectrophotometric analysis to give a resultthat the composition thereof could be expressed by the formula (Bi₀.40Eu₀.07 Y₀.53)₃ (Fe₀.94 Ga₀.06)₅ O₁₂. This oxide garnet single crystalafter removal of the substrate was polished on the surfaces and then cutinto a piece of 2.9 mm by 2.9 mm square having a thickness of 0.205 mm,which was examined for the magneto-optical characteristics in the samemanner as in Example 1. The results of the magneto-optical measurementwere: angle of Faraday rotation 45.2°; loss by light absorption 0.12 dB;and temperature dependency of the angle of Faraday rotation 0.11°/°C.

What is claimed is:
 1. A single crystal of an oxide garnet having achemical composition expressed by the formula

    (Bi.sub.a Eu.sub.b Ln.sub.1-a-b).sub.3 (Fe.sub.1-c M.sub.c).sub.5 O.sub.12,

in which Ln is a rare earth element other than europium, M is an elementselected from the group consisting of aluminum, gallium, indium andscandium, the subscript a is a positive number defined by 0.15≦a≦0.6,the subscript b is a positive number defined by 0.01≦b≦0.16 and thesubscript c is a positive number defined by 0.01≦c.≦0.1.
 2. The singlecrystal of an oxide garnet as claimed in claim 1 wherein the rare earthelement denoted by Ln is terbium.
 3. The single crystal of an oxidegarnet as claimed in claim 1 wherein the subscript c is a positivenumber in the range from 0.05 to 0.1.
 4. The single crystal of an oxidegarnet as claimed in claim 1 wherein the element denoted by M isgallium.